An oxidase is an enzyme that catalyzes an oxidation/reduction reaction. In plant cells, some oxidases use oxygen to oxidize organic substances.

In microbiology the oxidase test is used as a phenotypic character used for identification of bacterial strains. Strains may either be oxidase positve or negative, typically the Pseudomonadaceae are OX+ and Enterobacteriaceae are OX-. Another example is the preliminary identification of Neisseria and Moraxella species, which are both oxidase positive Gram-negative diplococci.

A positive oxidase reaction reflects the ability of a microorganism to oxidize certain aromatic amines, such as tetramethyl-p-phenylene diamine (TPD), producing colored end products (usually purple).